The present invention relates to a process for the production of synthetic resins containing hydroxyl groups. In an another aspect, this invention relates to baking varnishes prepared by blending melamine with a solution of the synthetic resin containing hydroxyl groups.
Coatings produced by this method are known, the vehicles of which consist of (1) a polyester comprising hydroxyl and carboxyl groups and (2) an aminoplast, e.g., a melamine resin. Such polyesters are composed of different carboxylic acids and of different specific compounds with hydroxyl groups. The properties of the coatings can be conformed to the individual use by selecting the individual components forming the polyester and by the ratio of the components employed. Esterification conditions are chosen such that the reaction is as complete as possible, i.e., until the acid number of the formulation is less than 5 mg KOH/g. The vehicle dissolved in a suitable solvent, optionally after the addition of the usual adjuvants, is applied onto the substrate and then baked. With polyesters of a very low acid number, acidic substances, e.g., p-toluene sulfonic acid, may be added to the lacquer.
Other copolymers, suitable as lacquer resin components for baking varnishes and containing hydroxyl groups, are produced from alpha, beta-unsaturated mono and/or dicarboxylic acids, one or more vinyl aromatic compound(s) and an alkylene oxide. When the unsaturated acid is dicarboxylic acid, then one of its carboxyl groups has been esterified with a monovalent alcohol or with a monoglycol ether.
In this connection, reference is made to Applicant's German Patent Applications Ser. Nos. P 21 37 239.4 and P 21 65 491.1 and to U.S. Pat. No. 3,838,087. In German Patent Aplication Ser. No. P 21 37 239.4 it is disclosed that the copolymerization of unsaturated dicarboxylic acid anhydride and a vinyl aromatic hydrocarbon in dispersion may be performed in the presence of a particular dispersant, thereby obtaining a product having a suitable K value. The K value is defined as a measure of the polymer chain length and is described in detail by K. Fikentscher in "Cellulosechemie", 13, 58 (1932). By performing all three process stages-copolymerization, halfesterification, and alkoxylation -- in the presence of a dispersant and in an extensively aromatic-free high-boiling hydrocarbon as reaction medium as more fully described in German Patent Application Ser. No. P 21 65 491.1, the suspension condition is maintained through all three process stages and the lacquer resin is obtained as a dispersion.
The hydroxyl copolymers of this invention, are, when used for baking varnishes, admixed in solution or dispersion with a melamine resin which serves as a crosslinking agent, and applied, after the addition of pigments, onto a substrate and baked at temperatures ranging from about 180 to about 220.degree. C.
The resulting lacquer films are, with respect to their mechanical properties and their resistance to chemical reagents, fully satisfactory although the field of application is restricted due to the high baking temperature.
Thus, the present invention, in one aspect is related to a process for the production of baking varnishes wherein the solution of a synthetic resin containing hydroxyl groups of this invention is blended with melamine resin as a crosslinking agent in a weight ratio of the hydroxyl copolymerizate to the melamine resin ranging from about 90:10 to about 70:30, and optionally, adjuvants, pigments, and an acid and characterized by employing as the synthetic resin containing hydroxyl groups, a copolymer of a vinyl aromatic hydrocarbon an ester of an alpha, beta-unsaturated monocarboxylic acid and an alpha, beta-unsaturated dicarboxylic acid in solution or dispersion, the carboxyl groups of which being extensively esterified with alcohols and/or glycolmonoethers, and reacted with alkylene oxides until the residual acid number of said hydroxyl copolymer reaches the value of from about 0 to 2 mg KOH/g of substance, based on a 50 percent solution or suspension, said solution or suspension then being adjusted to an acid number of at least 5 by adding phosphoric acid thereto after which the thus-formed varnish may be applied and baked, subsequent to admixing melamine, and optionally with pigments and adjuvants.
In preparing the novel copolymers of this invention preferably the polymerization reaction is conducted in the presence of a dispersing agent such as an ester of alpha beta-unsaturated dicarboxylic acid with an aliphatic alcohol of from 10 to 20 carbon atoms, etc. as exemplified by an isobutylene-maleic acid half-ester copolymer with a mixture of straight chain C.sub.12 to C.sub.18 alcohols as the alcohol component, etc. and in the presence of an aromatic hydrocarbon solvent boiling in the range of about 150 to about 190.degree. C. If desired, a portion (i.e., from about 10 to about 15 percent by weight) of the aromatic hydrocarbon solvent may be replaced by a kerosene cut boiling in the same range. The copolymerization reaction is conveniently carried out at temperatures ranging from about 60.degree. to about 170.degree. C. Preferably, the copolymerization is conducted in the presence of a free radical type catalyst such as an organic peroxide as exemplified by benzoyl peroxide, tertiary butyl hydroperoxide, methyl ethyl ketone peroxide, cumene hydroperoxide, etc.
After the initially formed copolymer of the vinyl aromatic compound, ester of the alpha, beta-monocarboxylic acid and the alpha, beta-dicarboxylic acid has been esterified with alcohols and/or glycol monoethers to yield the half-ester copolymer, an alkylene oxide or mixture of alkylene oxides is reacted at a temperature of about 60.degree. to about 100.degree. C utilizing a mole ratio of the alkylene oxide to the half-ester copolymer of about 0.1:1 to 1:1 or more.